A main wing of an aircraft has a hollow structure with wing surface panels forming wing surfaces mounted above and below spar materials. In general, this main wing has an inner space used as a fuel tank.
Further, in order to perform an inspection and maintenance operation and other operations on the inside of the fuel tank, an opening is formed on a surface of the main wing. At normal time, this opening is closed by an access door. When an inspection and maintenance operation or another operation is performed, the access door is opened.
The access door is configured of a door body placed on an inner space side of the main wing with respect to the opening and a clamp ring placed on an outer side of the main wing.
The door body and the clamp ring each have an external dimension larger than that of the opening. With the edge part of the opening being interposed by the outer peripheral part of the door body and the outer periphery part of the clamp ring, the door body and the clamp ring are fastened together with fastener members or the like, thereby closing the opening by the door body.
In an aircraft, there is a possibility of occurrence of an arc or spark at the time of lightning. Thus, to prevent the ignition of flammable gas in the fuel tank, a reliable lightning-protection measure is required at a joint portion between the access door and its surrounding opening on a main wing side.
To this end, a configuration is conventionally adopted in which a ring-shaped insulating material is interposed between the outer peripheral part of the door body placed on the inner side of the main wing and the opening on a main wing side (for example, refer to Patent Documents 1 to 3). In this configuration, with the insertion of the insulating material, the outer peripheral part of the door body and the main wing side are prevented from being electrically in direct contact with each other, thereby preventing an arc or spark from occurring from this portion into the fuel tank.